Add API paging support for bb queries #33

The prototype search API supports searching for all records within a bounding box via the GeoCouch extension to CouchDB:
https://github.com/couchbase/geocouch

The extension doesn't support paging, but does support a count=true option that returns the count of records within a bounding box (without returning the results).

A global bounding box could potentially return millions of VertNet records. Probably need to constrain how big a bounding box can be to avoid that. Could also constrain based on the count of records in a bounding box (e.g., if count is too high, it won't attempt to pull them from CouchDB). Given some upper count bound, implementing paging across results on App Engine side would be trivial.